Add MAPI support to allow Store apps send custom emails using a preferred email client

[xyzzer]

Proposal: Add MAPI support to allow Store apps send mail

Summary

Create an API similar to MAPI to allow Store apps to interact with mail apps like Outlook to share their content via email, allowing API users to define rich email body with inline images, file attachments, subject, to, cc, bcc fields that users can then edit in their MAPI client of choice before sending the email from their own email address.

Rationale

Since the creation of WinRT - Store app developers who wanted to allow sharing content from their apps by email were limited to using a web service or the share contract, which isn't sufficient for rich email content. An app like Snipping Tool allows its users to provide a button that with one click enables sharing an image with their mail client of choice. Meanwhile, its UWP replacement, Snip & Sketch is limited to using the share contract, which requires selecting a share target app and doesn't allow prefilling any mail fields. It's also limited to mail apps that support the mail contract.